I've been thinking a lot lately about the ratio of positive to negative interactions a couple having (10:1 being preferable) predicting "94%" of divorces according to Dr. Gottman's research (cited in Before Happiness).
I think it's easy to cross the two fences with plenty of positive interactions, 5:1 -- but when you get to that third fence, and you disagree, it becomes a nagging thing.
Every holiday, you have the same disagreement, and suddenly your positive interactions have dropped to 3:1 or 1:1 or worse. That's the danger zone, and typically one or both partners needs to double down on positive interactions. In my relationships, I've never really caught on to that dynamic.
But that's the critical junction to make the choice: stay or leave. BUT if you want to stay, you better buy flowers 5x this week, write love letters, and do tons of great stuff -- because you have to keep up the positive interactions while working through that last fence.